1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a processing method for magnetic resonance imaging (MRI) collection data that reconstruct images of an object using a nuclear magnetic resonance signal, which is generated by magnetic resonance of an atomic nuclear spin in the subject. In particular, the invention relates to a magnetic resonance imaging apparatus and a processing method for an MRI data collection that quickly displays confirmation images for confirming the usefulness of collected parallel imaging (PI) MRI data prior to the lengthy process required for complete MRI processing of the PI MRI data.
2. Background
Conventionally, as a monitoring apparatus at a medical treatment site, a magnetic resonance imaging (MRI) apparatus 1 as shown in FIG. 12 is used (see, for example, JP-A-2003-334177).
The MRI apparatus 1 is an apparatus that forms gradient magnetic fields in X axis, Y axis, and Z axis directions using respective gradient magnetic field coils 3x, 3y, and 3z of a gradient magnetic field coil unit 3 in an imaging area of a patient P, who is set in a cylindrical magnet for static magnetic field 2 forming a static magnetic field, and transmits an RF signal of a Larmor frequency from a radio frequency (RF) coil 4 to thereby cause an atomic nuclear spin in the patient P to magnetically resonate and reconstruct images of the patient P using a nuclear magnetic resonance (NMR) signal generated by excitation.
In short, a static magnetic field is formed inside the magnet for static magnetic field 2 by a static magnetic field power supply 5 in advance. Then, according to an instruction from an input device 6, a sequence controller control unit 7 gives a sequence, which is control information for a signal, to a sequence controller 8. The sequence controller 8 controls a transmitter 10, which gives an RF signal to a gradient magnetic field power supply 9 connected to the respective gradient magnetic field coils 3x, 3y, and 3z and the RF coil 4, in accordance with the sequence. Consequently, a gradient magnetic field is formed in the imaging area and the RF signal is transmitted to the patient P.
In this case, an X axis gradient magnetic field, a Y axis gradient magnetic field, and a Z axis gradient magnetic field formed by the gradient magnetic field coils 3x, 3y, and 3z are mainly used as a gradient magnetic field for phase encoding (PE), a gradient magnetic field for readout (RO), and a gradient magnetic field for slice encoding (SE), respectively. Consequently, an X coordinate, a Y coordinate, and a Z coordinate, which are positional information of atomic nuclei, are converted into a phase, a frequency, and a position of a slice of atomic nuclear spin, respectively, and the sequence is executed repeatedly while the phase encode amount is changed.
Then, an NMR signal, which is generated following the excitation of the atomic nuclear spin in the patient P, is received by the RF coil 4 and given to a receiver 11 to be converted into digitized raw data. Moreover, the raw data is fed to the sequence controller control unit 7 via the sequence controller 8. The sequence controller control unit 7 arranges the raw data in a K space (Fourier space) formed in a raw data database 12. Then, an image reconstructing unit 13 executes Fourier transformation for the raw data arranged in the K space, whereby a reconstructed image of the patient P is obtained.
As one of fast imaging techniques using such an MRI apparatus 1, there is a parallel imaging (PI) method (e.g., see the thesis “Carlson J. W. and Minemura T., Image Time Reduction Through Multiple Receiver Coil Data Acquisition and Image Reconstruction, MRM 29: 681–688, 1993”, the thesis “Sodikson D. K. and Manning W. J., Simultaneous Acquisition of Spatial Harmonics (SMASH): Fast Imaging with Radiofrequency Coil Arrays, MRM 38:591–603, 1997”, the thesis “Pruessman K. P., Weiger M., Scheidegger M. B., and Boesiger P., SENSE: Sensitivity Encoding for Fast MRI, MRM 42:952–962, 1999”, and the thesis “Ra J. B. and Rim C. Y., Fast Imaging Using Subencoding Data Sets From Multiple Detectors, MRM 30:142–145, 1993”). The PI method is a imaging method in which a multi-coil constituted by plural surface coils is used as the RF coil 4 and an NMR signal is received in the respective surface coils simultaneously to reconstruct an image. According to the PI method, since it is possible to reduce the number of phase encodes necessary for reconstruction of an image by the number of surface coils, imaging time can be reduced.
According to the PI method, imaging time with high resolution of a few seconds is possible even in three-dimensional (3D) imaging. Thus, the PI method is applied to dynamic imaging such as a magnetic resonance angiography (MRA) method for injecting a contrast agent into the patient P and observing temporal movement of the contrast agent. In dynamic imaging according to the PI method, since imaging in an extremely large number of temporal phases is possible, it is possible to image movement of the contrast agent in detail in a contrast MRA method.
Imaging according to the PI method is carried out in a procedure shown in Fig. 13. First, in step S1, as described above, imaging according to the PI sequence is performed and raw data is arranged in the K space formed in the raw data database 12. In the case of 3D dynamic imaging, 3D volume data as shown in FIG. 14(a) is obtained for respective temporal phases T.
Moreover, in step S2 in FIG. 13, the image reconstructing unit 13 reconstructs images for matrixes designated in advance in the K space. As shown in FIG. 14 (b), raw data included in the matrixes are subjected to 3D Fourier transformation (3D-FT), whereby 3D image information is obtained.
Here, aliasing called folding occurs in images obtained according to the PI method. Thus, in step S3 in FIG. 13, a PI unfolding processing unit 14 executes unfolding processing as post processing of the reconstructed image on the basis of a difference in sensitivity of the respective surface coils of the multi-coil.
As shown in FIG. 15, sensitivity map data D1 of the multi-coil is collected in advance and stored in a sensitivity map database 14a shown in FIG. 12. Then, as shown in FIG. 15, the P1 unfolding processing unit 14 slices sensitivity map data D1, into alias which respectively correspond to respective slice surfaces of 3D image information (actual space data) D3. PI unfolding processing is obtained by subjecting raw data D2 in K space to 3D-FT, and using sensitivity map database 14a as sensitivity map data D4 for unfolding processing. Moreover, concerning the 3D image information D3, the PI unfolding processing unit 14 executes PI unfolding processing for respective slices using sensitivity map data D4 for unfolding processing to obtain 3D images after PI unfolding processing as unfolded images D5.
As a result, as shown in FIG. 14 (c), all images in all temporal phases are obtained and stored according to circumstances.
In step S4 in FIG. 13, with an image in a predetermined temporal phase as a parent image, a difference processing unit 15 executes difference (complex difference, absolute value difference) processing for images in temporal phases later than that of the parent image as required. Consequently, as shown in FIG. 14 (d), difference images in temporal phases later than that of the parent image are obtained and stored according to circumstances.
Moreover, in step S5 in FIG. 13, an MIP processing unit 16 executes MIP processing, which is image processing according to a maximum intensity projection (MIP) method, as required to project all the 3D images and the difference images on a 2D plane and obtain MIP images. In other words, slice image data having a maximum signal value among respective slice image data forming the 3D images is set as a value on the projected surface, whereby all the 2D images shown in FIG. 14 (e) or the MIP images, which are difference images, shown in FIG. 14 (f) are obtained.
As a result, in step S6 in FIG. 13, the MIP images and all the images and the difference images in all the temporal phases are stored in an image database 17 as images for confirmation. Moreover, an image display unit 18 gives the images for confirmation stored in the image database 17 to a display device 19 and displays the images for confirmation. Consequently, an operator can judge propriety of imaging by confirming the images for confirmation such as the MIP images and all the images in all the temporal phases.
On the other hand, in 3D imaging by the MRI apparatus 1, in view of the fact that 3D reconstruction processing for 3D volume data is enormous, a technique for creating a Reference View is used (see, for example, U.S. Pat. No. 5,166,875 and JP-B-5-78341). The Reference View is a 2D image that is obtained by slicing two-dimensional (2D) data in the center of the K space from 3D volume data and subjecting the 2D data to the 2D-FT and is briefly equivalent to a projected image. With this Reference View, the operator can confirm an image with a smaller amount of data processing and in a short time.
In addition, there is proposed a technique for, in the 3D imaging by the MRI apparatus 1, subjecting 2D data obtained from time to time during imaging to the 2D-FT for speedup of image display to thereby reconstruct and display a 2D image (see, for example, JP-A-2-46828).
In fast contrast MRA imaging according to the PI method, observation on a real time basis with improved imaging time resolution is desired. However, in the conventional MRI apparatus 1, since time of about several tens seconds to several minutes is required for processing such as image reconstruction including reconstruction processing like zero padding and the PI unfolding processing, observation on a real time basis is difficult. Therefore, long time is required until an image for confirmation, which is a final image, is displayed, and the operator cannot confirm propriety of imaging immediately after the imaging. As a result, there is a problem in that it is impossible to let the patient P off the MRI apparatus 1 until an image for confirmation is obtained and the operator confirms propriety of imaging.
Moreover, in the case of dynamic imaging, since imaging an extremely large number of temporal phases is usually performed, there is a problem in that the volume of images to be stored becomes enormous and it takes time to retrieve and transfer images.
On the other hand, the conventional technique using the Reference View is not applicable to PI because even if the Reference View is used directly, such images would have aliasing in a phase encode direction. Thus, only an image of accuracy insufficient for confirming propriety of imaging can be quickly obtained conventionally with PI. In other words, in PI imaging, since a field of view (FOV) in the phase encode direction is set small, when quick reference images are reconstructed in that state, the images will have aliasing in the phase encode direction (according to the FOV size).